Approved Final TMDLs

Final TMDLs Approved by EPA: Lake Lariat

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Lake Lariat is an impoundment located near Cherry Hill in southern Calvert County, Maryland. The impoundment, which is owned by the Chesapeake Ranch Estates (community association), lies in the Patuxent River watershed. An earthen dam was constructed in 1965. The impoundment is currently used for recreational activities.

Based on mercury data in fish tissue from a subset of lakes across the State, the Maryland Department of Environment (MDE) announced a statewide fish consumption advisory for lakes this year. This advisory has been established statewide as a precautionary measure because the primary source of mercury is understood to be atmospheric deposition, which is widely dispersed. Based on additional fish tissue data, Maryland has verified that Lake Lariat is impaired due to mercury in fish tissue.

Section 303(d) of the federal CWA and EPA’s implementing regulations direct each state to identify and list waters, known as water quality limited segments (WQLSs), in which current required controls of a specific substance are inadequate to achieve water quality standards. The CWA requires the development of a Total Maximum Daily Load (TMDL) for all impaired waters on their Section 303(d) list. A TMDL reflects the maximum pollutant loading of an impairing substance a waterbody can receive and still meet water quality standards. A TMDL can be expressed in mass per time, toxicity or any other appropriate measure (40 CFR 130.2(i)). A TMDL must take into account seasonal variations, critical conditions and a margin of safety (MOS), to allow for uncertainty. Maryland’s 2002 proposed 303(d) list prepared by MDE lists Lake Lariat as impaired for mercury (watershed 02131110).This TMDL was developed in response to the determination of impairment.